SOLAR SYSTEM FAMILY-FACTS ABOUT THE MOON, OUR LITTLE CELESTIAL NEIGHBOUR

 Although mankind succeeded in obscuring the stars under a blanket of artificial light and smog, the moon always finds some way or the other to catch our depleting sense of admiration. Those who have walked a lonely path under a full moon can surely feel the intoxicating nature of our celestial companion. Just like the Sun, our moon, sometimes called Luna, has stood witness to the whole journey of man's evolution from chimps. It was under this moon where our ancestors told and retold stories about the valour of heroes and gods when they slayed their demons. The night sky came alive with mythological beings, and with time, man became capable of exploring the sky and beyond. And finally the whole of mankind conquered the lunar soil when Neil Armstrong set his foot for the first time in 1969.

A Bit of Mythology- Like Sol, Luna also occupied a significant place among the early civilizations, sometimes being worshipped as a male or a female deity. The early Greeks worshipped our celestial companion as their goddess ''Selene'' and later associated her with ''Artemis''. According to their beliefs, Selene drove across the night sky in a horse drawn chariot, just like her brother ''Helios''- the sun god. On the other hand, the Romans worshipped the Moon as the goddess ''Luna'' and later as ''Diana''. The ancient Indians worshipped the moon as god ''Chandra'' or ''Soma'' and has many interesting myths about its motion across the night sky. Other cultures that worshipped the moon, either in the form of a god or goddess include the Polynesian Islanders, The Maori tribe of Africa, the Aztecs, the Inuit, the Chinese, and the Egyptians.    

Basic Facts- The Moon is Earth's only natural satellite, preferably formed from the collision of protoplanetary masses during the early period of our solar system's formation. Its composition is nearly similar to that of Earth, with an equatorial diameter of 3448 km and lacks any sort of atmosphere, magnetosphere or a hydrosphere like its parent planet. The Moon hasn't got any light of its own, and shines by reflecting sun-light.

A Full Moon/Image Credits:Photo by form PxHere

Characteristics:

• Orbital & Rotational Characteristics: The  Moon orbits Earth at an average distance of 384,000 km, with an orbital eccentricity of 0.055 and an orbital period of roughly 29 days. The apogee distance remains at 406,700 km and the perigee is about 356,400 km. It is also in a synchronous rotation, as the time taken to rotate about its axis is the same compared to the time required to orbit Earth. As a result of this sort of celestial motion, the Moon always has the same face pointed towards us. The face that we see every-time is called the ''near side'' and the other side is popularly called ''dark side'' or ''far side'' in astronomical language. However, that side isn't technically dark but is blasted with solar radiation and visible light during its new moon phase. It is good to mention that the far side is totally different from the near side, as the former is heavily cratered and is not at all smooth with dark patches(Lunar Maria) which are found over the near side.   

• Internal Structure: Since the Moon is a terrestrial body like Earth, it has definitely got a distinct crust, a mantle, a fluid outer core and a solid iron-rich inner core. It is composed of silicon, aluminum, calcium, iron, magnesium, titanium and sodium, particularly in the form of their respective oxides. With an equatorial radius of 1738 km the moon is the 2nd densest natural satellite, having an average density of 3.34 g/cc after Io(A Jovian moon; density of 3.58 g/cc). However, the composition of our little companion isn't fully understood by scientists and recent studies suggest that the Moon is a bit rusty with more concentrations of iron oxides than previous estimates.

• Surface Geology: 31% of the near side is partly smooth with dark plains called ''Lunar Maria''(Maria is the Latin for ''seas'') while the rest is made up of highland plains called ''terrae''. On the contrary, the far side is heavily cratered due to asteroid and meteoroid impacts. The dark patches which are clearly visible by our naked eyes, once mistaken as seas by early astronomers are nothing but solidified basaltic lava plains formed from ancient volcanic activity. The lunar craters range in diameter from a few meters to the largest one being the South Pole-Aitken Basin(diameter=2,240 km). It is also the second largest impact basin in the Solar System. Other notable impact craters include the Procellarum, a lunar Maria with a diameter of 3,000 km and is also an unconfirmed impact crater, the Imbrium(diameter=1146 km), the Serenitatis(diameter=674 km), the Crisium(diameter= 556 km), the Smythii(diameter=373 km) and the Orientale(diameter=294km). The lunar surface also contain some fault scarps which indicate that this celestial body has shrunk in size since its birth, just like Mercury. Lack of any recent volcanic eruptions in terms of geological timescales suggest that the Moon is volcanically inactive. However, some seismic activity is still going on and an explorer there could experience moonquakes. The Lunar surface is again covered by a very thin layer of fine dust grains called ''regolith'', which is primarily composed of oxygen. silicon, calcium, magnesium, aluminum and traces of other elements.

Heavily Cratered Lunar Surface/Image Credits:Pixabay

• Surface Temperature & Presence of Water Ice: Absence of any sort of atmosphere has forced our little neighbour to witness extreme high and low temperatures. Daytime temperatures near the equatorial regions typically rise to about 400K(120℃), while the nighttime temperatures fall to 140K(-130℃). The poles however are not exposed to direct sunlight and some regions remain under perpetual darkness. This data made scientists speculate about the possibility for the existence of water ice. The first evidence of water ice came from the observations by the Clementine Spacecraft, radar observations from the Arecibo Observatory and of course from analyzing the rock samples brought by the Apollo Missions. In 2008, Chandrayaan-1 spacecraft, sent by the Indian Space Research Organization also confirmed the presence of ice near the polar regions.

• A Very Thin Atmosphere: The Lunar atmosphere is so thin and rarefied that sometimes it's considered to be near vacuum. However, there are some gaseous molecules of Argon, Helium, Neon, Sodium, Potassium, and Hydrogen that still linger around the Moon's periphery. But scientists speculate that our celestial neighbour have had a relatively dense atmosphere for about a period of 70 million years and that too between 3-4 billion years ago. 

• Magnetic Field: The Moon does not have a global dipolar magnetic field like Earth, which is generated from a geodynamic circulation of molten metal below the surface. The lunar magnetic field results from crustal magnetization which suggests that the Moon might have had a global magnetic field in its past, which too got shut down due to the cooling and solidification of the inner core. 

History of Formation- The formation of our moon has three main hypotheses, of which only one has been widely accepted as a sound theory. But this topic is still undergoing further debates as there are certain lunar features that can't be explained by a single theory. They are as follows:

1. Giant Impact Hypothesis- This states that a large celestial body, probably the size of Mars collided with the newly formed Earth. This collision threw out a large part of Earth's mass, which then under the action of gravity coalesced to form the Moon.
2. Co-formation Theory- Natural satellites(here our Moon) can also form alongside their respective planet's formation from the coalition of gas and dust. During their formation, gravity accumulated clumps of matter together, and the larger clump became the parent planet while the smaller one became it's satellite.
3. Capture Theory- This one states that the moon formed somewhere else within the solar system. As it was passing through the vicinity of our early Earth, that rogue body got captured by latter's gravity and started revolving around the planet. 

The Dark Side is Not Always Dark/Image Credits:Apollo 16 astronauts, Public domain, via Wikimedia Commons  

Generation of Tides- The Earth-Moon System exerts mutual gravitational forces or tidal forces on each other in addition to a smaller amount from the Sun. These tidal forces are manifested in the form of high tides and low tides on Earth's oceans, when the water level periodically rises and falls. As the Earth rotates around its axis, the side facing the Moon experiences high tides, while the adjacent side experiences low tides. Thus there are two low tides and two high tides over a period of 24 hours. The Sun and the Moon together also causes spring tides and neap tides on Earth. Spring tides occur when the Sun, Moon and Earth all lie in a straight line configuration during New Moon and Full Moon, and the tide is at maximum intensity. Neap tides occur when the Moon is at first quarter or third quarter, and the Sun-Earth-Moon is at a 90° angle. During neap tides, the magnitude of water level rise is at its minimum as the Sun and the Moon both cancel out their tidal forces on Earth.

Observations & Explorations: 

• Observations: The lunar observations(in this article) has been subdivided into three distinct categories as listed below:
  

Solar Eclipse/Image Credits:Brocken Inaglory, CC BY-SA 3.0, via Wikimedia Commons

1. Observations of Lunar Phase- Firstly, the Moon does not have any light of its own, but shines by reflecting infalling sun-light. In spite of being the brightest object of the night sky, the Moon has got a very low albedo, where albedo is defined as the amount of light reflected from a surface without getting absorbed by the reflecting surface. Thus, the Moon is not a very good reflector, but it appears brighter just because of its sheer size and close proximity. Secondly, the percentage of illuminated surface varies as the Moon waxes and wanes during its revolution around Earth. A new-moon occurs when the Moon becomes aligned with the Sun on the same side of Earth(note-this alignment is not an eclipse). During this phase its far-side gets illuminated by the Sun and the near-side(which we always see) remains in total darkness. A full-moon occurs when the moon aligns with the Sun on the opposite side of Earth. And, during this phase the near-side is completely illuminated by the Sun. The period of waxing(increasing luminosity) occurs as the Moon moves from a new-moon phase to a full-moon phase; the period of waning(decreasing luminosity) occurs as it moves from its full-moon phase to a new-moon phase. This change of phase occurs every month, and we have a new moon every 29.5 days.
2. Observation of Eclipses- The Moon is smaller compared to Earth, and is much much smaller than the Sun. But in our skies both of them has nearly the same apparent dimensions. Solar eclipses occur when the Moon moves in between the Sun and Earth, in a syzygy configuration(a straight-line configuration between three or more celestial bodies) and blocks the sunlight from reaching Earth. As a result some places on Earth comes directly under the Moon's shadow. A total solar eclipse occurs when the Moon completely covers the Sun, whereas an annular solar eclipse occurs when the Moon falls short to completely cover the Sun and observers on Earth can see a ring like the above image. Lunar eclipses occur in a similar way, where the Earth comes in between the Sun and the Moon. The latter comes under Earth's shadow and we experience partial or total lunar eclipses as shown below.
3. Beginning of Astronomy- The Moon holds a special place in the development of early astronomy and of course time keeping, as the notion of months came from observations of lunar phases since ancient times. The Babylonians, Chinese, Indians and the Greeks observed the Moon night after night and derived some important conclusions about the lunar cycle, its spherical shape and also speculated about the possible reasons for its brilliant silvery light. It's good to note that Ptolemy, Aristarchus, and Archimedes tried to calculate the distance between the Earth and Moon and were quite accurate subjected to their times. During the middle ages, there was a misconception that the Moon was perfectly smooth and without undulations, ridges or craters in it. But it was Galileo Galilei who debunked this misconception when he trained his newly invented telescope towards the Moon in 1609. He then revealed that the glorious Luna has got a rather scarred surface.

Lunar Eclipse/Image Credits:Pixabay

• Explorations: Lunar explorations are quite easy compared to other distant worlds. And that's why during the period from 1955-1975, the whole world witnessed mankind's greatest achievement-the explorations of a world other than ours and the landing of a human being on the lunar soil. The lunar explorations are as follows:

1. The Soviet Union was the first nation to attempt unmanned explorations with their ''Luna Program'' between 1959-1976. Luna-1, launched on 2nd January 1959, became the first spacecraft to escape the Earth-Moon gravitational system and complete the first Moon flyby, while Luna-2, launched on 12th September 1959, became the first man-made object to impact the lunar soil. The first successful lunar flyby was done by the Luna-3, launched in October 1959, and provided first-time images of the far side that remains hidden from Earth. The Luna missions also placed some orbiters like the Luna-10 which became the first artificial satellite of the moon, launched on 1966. Luna-16 became the first robotic sample return from beyond Earth. 
2. The Soviet ''Lunokhod Program'' that lasted between 1969-1977 comprised of rovers to land and explore the lunar surface. Lunokhod-1, launched on 10 November 1970, and carried to the moon by the Luna-17 spacecraft became the first man made object to wander over the lunar surface. The Russians also became successful in soft landing and sample return with their Luna-16, 20 and 24 missions.
3. The United States of America also attempted countless unmanned lunar explorations until they became the first nation to land a human being on the moon. The first attempts were made with the impactor probes of the ''Ranger Program'', that lasted between 1961-1965. These impactor probes were designed to crash land on the lunar surface and provide high resolution images during the final moments before impact. The next phase started with the ''Lunar Orbiter Program'', a series of unmanned orbiters that were launched between 1966 and 1967. These were primarily entrusted with the task of mapping possible sites for a manned mission.
4. The first soft landing by the United States was completed with the ''Surveyor Program'' between 1966 and 1968. Finally after all these missions the United Nations with their ''Apollo Program'' became successful in crewed missions to the moon. The Russians however, never succeeded to land a man on the moon and they finally abandoned their efforts. 
5. All of the above space probes, landers, orbiters and impactors have analyzed the lunar geography, soil structure and composition, internal structure and composition, magnetic field, gravitational field, and returned other scientific data. 
6. There are also some very recent missions to the moon that include the ''Chandrayaan-1 & 2 '' from India, ''Chan'ge'' from China, ''Beresheet'' from Israel, ''Lunar Reconnaissance Orbiter'', ''LCROSS'', ''Clementine'', ''Gravity Recovery and Interior laboratory'' or GRAIL, ''Artemis'' and others from the United Nations, ''Kaguya(Selene)'' from Japan, and ''Smart-1'' from Europe.  

Man on the Moon, Neil Armstrong/Image Credits:Pxhere

Armstrong's Footprint and other Apollo Missions- The ''Apollo Program'' was the most important mission in the age of space explorations between 1961-1972. Alhough there were some unsuccessful attempts and few lives were lost during the Apollo-1, nevertheless, Apollo-8 became the first crewed mission in an orbit around the Moon. Astronauts Frank Bormann, James Lovell and William Anders sent back iconic pictures of the lunar surface, with the Earth rising slowly above the lunar horizon on December 24, 1968. This was followed by the Apollo-9 & 10 that demonstrated the United States' capability of landing a man on the Moon. Finally on 20th June, 1969, Neil Armstrong became the first man to set a foot upon the lunar soil and was soon followed by Buzz Aldrin, while Michael Collins remained in a command module on the lunar orbit. The Apollo-11 mission was succeeded by five other manned missions, the Apollo-12, 14, 15, 16, and 17. During these missions other groups of astronauts performed a variety of scientific experiments and subsequently left numerous man made objects to stay on the lunar surface for a long time, including magnetometers, seismometers, retro-reflectors, a bunch of flags, a plaque, and some footprints. Since, the moon hasn't got any winds, due to the absence of an atmosphere, so Armstrong's footprint is destined to remain there forever, for millions of years, along with the footprints of other astronauts. 

''That's one small step for a man, one giant leap for mankind''-Neil Armstrong/Image Credits:Pixabay 

Very Distant Future- The last crewed mission to the moon was the Apollo-17 in 1972, and there had been a significant slump in interests for 50 years. But very recently, NASA has unveiled their plans(Artemis Program) for a new generation of crewed lunar missions. Other countries viz., Russia, India, China and the European nations have also aimed at the Moon. So in the near future, it is expected that this little satellite might(or must) become a second home for all mankind. The moon is the only celestial body closest to Earth, at an average distance of 384,000 km and that's why it's quite easy to reach there. Assumptions based on our present technological advancements makes us capable of setting up lunar bases and colonies by the end of 2040s. The lunar base would accelerate manned explorations of distant worlds like Mars, a few asteroids and probably some of the Jovian moons before the culmination of this century. The Moon can also serve as an excellent place for setting up radio or optical telescopes in some of the craters that remain in perpetual darkness, and also away from the radio chatter or atmospheric turbulence on Earth. 

                                        It must also be mentioned that the Moon is moving away from Earth at a rate of 4 cm per year, and as a result a day on Earth is getting longer, roughly by 1.4 milliseconds every 100 years. Because of the tidal drag that the Moon exerts on our planet, the latter's rotation is also getting slower and in about 50 billion years, both of them would become tidally locked with each other. This is only possible if our Earth survives the rapid expansion of the Sun as the latter slowly gravitates towards its red giant phase in about 5-7 billion years from now.  

Earth & Moon/Image Credits:commons.wikimedia.org

Some Quick Facts!

References:

1. https://solarsystem.nasa.gov/moons/earths-moon/overview/

2. https://en.wikipedia.org/wiki/Moon

3. https://moon.nasa.gov/exploration/moon-missions/